Method of hardening a case hardened steel



United States Patent Offiee 3,Ml,80l Patented July 21, 1964 3,141,801 METHOD OF HARDEgrlllglG A CASE HARDENED The invention relates in general to the method of hardening steel and more particularly to the method of, first, case hardening the steel and, second, hardening the core.

In the art of hardening steel it has long been customary to provide a hardening by heating to a suitable temperature and quenching, plus a suitable draw or temper, to achieve substantially uniform hardness throughout the steel. Subsequently, the steel was case hardened, as by nitriding to" achieve an even harder surface. Nitriding is a slow process taking from 40 to 100 hours in the usual case at elevated temperatures which are supposedly below the lower transformation point of the steel. However, it has been found that with tool steels which are capable of being hardened to a considerable degree, this normal process of hardening the entire steel and then case hardening the surface to obtain increased hardness thereat has the effect of softening the core somewhat. In a particular case the core softened about 7 to 10 points on the Rockwell C scale which is sufficient to make a core that has decreased compression strength. When such a steel is used in a die subject to considerable compressive stress, the die core collapsed or crushed, and the surface form is fractured and breaks off, thus ending the useful life of the die.

Accordingly, an object of the invention is to eliminate the above disadvantages by providing a hardened steel with a hardened case plus a high strength core.

Another object of the invention is to provide a method of nitriding or other case hardening to harden the case first and then subsequently to harden the core.

Still another object of the invention is to provide a method of case hardening to obtain a specific surface hardness and then following with a hardening procedure to harden the entire steel throughout.

It will be understood that ferrous alloys capable of being hardened by nitriding or other case hardening procedures are characterized by extreme hardness when so processed. A preferred embodiment of the invention may be practiced on SAE D- steel which is a cobalt alloyed high carbon, high chromium steel. This steel is subjected to a nitriding step by heating in a nitrogen atmosphere such as being subjected to ammonia gas at a suitable temperature. A temperature in the range of 900-1000 F. for approximately 72 hours will obtain a case hardness of about .015 to .020 inch deep with a surface hardness of about 68-70 Rockwell C. The core will be relatively soft, namely, about the same hardness as the annealed steel with which one started. This will have a hardness of about 19-22 Rockwell C. The steel is then soaked or preheated in a neutral atmosphere controlled furnace to a temperature of about l250l300 P. which is below the lower transformation point or lower critical temperature of the steel. It is soaked at this temperature for one hour per inch of thickness to assure that the steel is soaked thoroughly at this temperature. The temperature is next raised to about 1870 F. and again soaked for one hour per inch of thickness. The steel is then quenched and this particular steel is an air quench steel. This hardens the steel all the way through. The steel is then tempered, preferably at a low temperature of about 300 F., again for one hour per inch of material thickness. This results in a steel having a surface hardness of about 64-65 Rockwell C and a core hardness of about 6061 Rockwell C. The above process results in a steel with a hard case and a hard core which successfully will resist compression forces and will have a compression strength in excess of 200,000 p.s.i.

Other objects and a fuller understanding of this invention may be had by referring to the following description and claims.

Die steels according to the present invention have been used in making thread rolling dies. Such dies are subjected to considerable compressive forces especially where high speed production is required. Each user of dies naturally desires to obtain as great a production capacity as possible and thus runs the dies at higher and higher speeds and with greater forces on the dies in order to turn out more.

pieces in a given length of time. The previous methods of hardening a die steel throughout and then nitriding or other forms of case hardening had the deleterious effect of destroying some of the hardness of the core. Nitriding produces a hard case and this is very often used for high surface hardness. In order to make the nitrides penetrate the surface some prior art processes have suggested raising the nitriding temperature. This is unsatisfactory because it pushes the temperature close to the lower transformation point of the tool steel. It has been found that the lower the temperature at which the nitriding can be accomplished, the greater the surface hardness. Yet the depth of the surface hardness is increased by increased nitriding temperatures even though this lowers the surface hardness. In the prior art process of hardening first and nltridmg second, I have found that this nitriding, even at the lower temperatures of about 9001000 F., accomplishes a form of spherodizing or process annealing which is not full annealing but is enough to give a lower compression strength. The present invention, therefore, contemplates case hardening the steel as a first step and hardening as a second step.

The nitriding in the preferred form of the invention is sufficient to obtain a hard case on the die or tool so as to obtain a very hard wearing surface. In such die steels which are capable of being hardened and nitrided this process is applicable. The process is especially applicable to cobalt alloyed high carbon, high chromium steels such as SAE D-S steel. This steel has an analysis of the following:

Percent Carbon 1.4 Chromium 13 .0 Cobalt 3 .3 Nickel .5 Manganese .3 Silicon .6 Molybdenum .6 Sulphur Low Phosphorous Low Iron Balance The nitriding is carried out at a temperature several hundred degrees below the lower transformation point in the range of 9001000 F. The lower transformation point is about 1390 F. for D5 steel. It is preferred that 975" F. be used. The nitriding is continued for a period of 40 to hours to obtain a hardened case about .015 to .020 inch deep. It is preferred that this nitriding be continued for about 72 hours. This nitriding may be carried out in any of the usual manners such as in a nitrogen atmosphere controlled furnace. This may be ammonia gas or any other suitable source of nitrogen. The surface hardness of the steel after such nitriding step is about 68-70 Rockwell C. The core hardness will be the same as the annealed steel before the nitriding step. This core hardness in the case of D-5 steel is about 25-27 Rockwell C. The tool steel may then be subjected to a hardening process which preferably is to soak the steel in a preheat step by heating in a neutral atmosphere controlled furnace or by pack hardening to eliminate normal atmosphere. This preheat step is to soak the steel thoroughly at a temperature below the lower transformation point and preferablytati100-15 F. below this lower transformation point. For D- steel this soaking temperature is about 1250-1300 F. This preheat soaking assures that the steel will be heated completely through to this temperature and, accordingly, is carried out for one hour per inch of material thickness.

The steel is next rapidly raised in temperature to the proper hardening temperature. This may be done by raising the temperature of the furnace or by quickly transferring the steel from one furnace at the preheat temperature to a second furnace at the proper hardening temperature. This hardening temperature is about 300- 350 F. above the upper transformation point. For D-5 steel this upper transformation point is about 1530 F., and the proper hardening temperature for maximum hardness is about 1870 F. The steel is soaked for one hour per inch of thickness to heat through at this temperature. The steel is then quenched and in the case of D-5 steel this is an air hardening or air quenching steel. This hardening step hardens the material throughout. The material is then preferably tempered or normalized at a lower temperature to relieve any hardening strains. It may be tempered at any temperature in a range safely below the lower transformation point which would include any temperature up to about 1000 F. However, it is preferred to temper at a low temperature of about 300 F., again soaking the material for one hour per inch of material thickness.

The hardening step of heating to about 1870 F. and soaking for a few hours, dependent on the thickness of the piece, does have the effect of dissipating the nitrides somewhat toward the core. However, this process is a slow one and the case hardness still is almost completely retained being dropped in hardness only a few points on the Rockwell C scale.

The above hardening process achieves a surface hardness which is only about 4 or 5 points Rockwell C below the surface hardness achieved on the nitriding step, namely, surface hardness of about 64-65 Rockwell C. The core hardness is only 3 or 4 points Rockwell C below that of the surface, namely, about 60-61 Rockwell C. This hard core is considerably above the hardness of the core obtainable by the standard procedure of, first, hardening and, second, nitriding. In that case the core hard ness is only about 52-55 Rockwell C, which is too soft to have a good compressive strength. As a result, with die steelshardened and then nitrided, as in the prior art process, dies would not stand up under high production schedules. Thread rolling .dies made according to the prior art process turned out about 3%. to 4 million pieces. The same type thread rolling dies made according to the improvednitriding and then hardening process described herein have turned out 16 to 18 million pieces. Thus the dies last about 5 times as long as before and permit much greater production capacity. The preferred process on D-5 steel results in a compression strength of the core of the die in excess of 225,000 p.s.i. This is sufficient to resist crushing by the high compressive stresses 4 encountered in high production thread rolling equipment and thus the life of the die is materially increased.

The nitriding and hardening process of the present invention is applicable to all forms of the D-5 steel, including cast and forged types. This steel in cast form is less dense and has a more open grain structure than when forged, and it has been found that the process of the present invention is slightly'superior on cast steel as compared to forged steel.

Although this invention has been described in its preferred method with a certain degree of particularity, it is understood that the present disclosure of the preferred method has been made only by way of example and that numerous changes in the details of the method may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.

What is claimed is:

1. The process of hardening a high chromium, high carbon die steel capable of being hardened and nitrided,

comprising the steps of nitriding the steel by heating in a nitrogen atmosphere at about 900 to 1000 F. for approximately 40-90 hours to obtain a nitrided hardened case .015 to .020 inch deep with a surface hardness of f. about 68-70 Rockwell C and a relatively soft core, subsequently raising the temperature to about 300 to 350 F. above the upper critical temperature for one hour per inch of material thickness, quenching the steel to harden the core thereof, and tempering the steel by heating to a temperature less than 600 F. for one hour per inch of materialthickness to obtain a surface hardness of about 64-65 Rockwell C and a core hardness of about 60-61 Rockwell C.

2. The process of hardening a high carbon, high chromium die steel containing about 3.3% cobalt, 1.4% carbon and 13% chromium, comprising the steps of nitriding the steel by heating in a nitrogen atmosphere at 975 F. for approximately 72 hours to obtain a case .015 to .020 inch deep with a surface hardness of about 68-70 Rockwell C '1 and a core hardness of about 19-22 Rockwell C, preheating the steel in a neutral atmosphere to about 1250- to 1300 F. for one hour per inch of material thickness, subsequently raising the temperature to about 1870 F. for one hour per inch of material thickness, air quenching the steel, and tempering the steel by heating to about 300 F. for one hour per inch of material thickness to obtain a surface hardness of about 64-65 Rockwell C and a core hardness of about 60-61 Rockwell C.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Transactions of the A.S.S.T., November 1928 (pages 738-739 relied upon). 

1. THE PROCESS OF HARDENING A HIGH CROMIUM, HIGH CARBON DIE STEEL CAPABLE OF BEING HARDENED AND NITRIDED, COMPRISING THE STEPS OF NITRIDING THE STEEL BY HEATING IN A NITROGEN ATMOSPHERE AT ABOUT 900* TO 1000*F. FOR APPROXIMATELY 40-90 HOURS TO OBTAIN A NITRIDED HARDENED CASE .015 TO .020 INCH DEEP WITH A SURFACE HARDNESS OF ABOUT 68-70 ROCKEWELL C AND AW RELATIVELY SOFT CORE, SUBSEQUENTLY RAISING THE TEMPERATURE TO ABOUT 300* TO 350* F. ABOVE THE UPPER CRITICAL TEMPERATURE FOR ONE HOUR PER INCH OF MATERIAL THICKNESS, QUENCHING THE STEEL TO HARDEN THE CORE THEREOF, AND TEMPERING THE STEEL BY HEATING TO A TEMPERATURE LESS THAN 600*F. FOR ONE HOUR PER INCH OF MATERIAL THICKNESS TO OBTAIN A SURFACE HARDNESS OF ABOUT 64-65 ROCKWELL C AND A CORE HARDNESS OF ABOUT 60-61 ROCKWELL C. 